The extrusion of thermoplastic materials as a means to produce a variety of end use products is a notoriously well known thermoplastic processing technique. Usually, thermoplastic stock in the form of pellets is introduced into a conventional screw extruder which thoroughly fluxes the thermoplastic in the extruder's barrel and discharges molten thermoplastic through an extrusion die. The extrusion die thus functions so as to shape the thermoplastic discharged from the screw extruder so as to achieve the desired product configuration (i.e., cross-sectional profile).
Difficulties may be encountered however during extrusion processing of products formed of low melt strength thermoplastics and/or when the products are intended to have rather complex cross-sectional profiles. One difficulty which may be encountered when low melt strength thermoplastics are extruded is the inability of such thermoplastics to be self-supporting when initially discharged from the extrusion die. This problem usually necessitates some additional support structure for the extrudate to allow it to cool (i.e., solidify) to an extent whereby it is then capable of supporting itself.
Other problems are attendant with the extrusion of low melt strength thermoplastics to form products having rather complex geometric profiles. Typically, extruded products having complex geometric profiles are produced using elongated extrusion dies which gradually shape the extrudate in an extrusion cavity defined between the die's inlet and outlet ends. That is, the extrusion cavity of these dies will usually have an inlet profile which roughly approximates the final cross-sectional profile of the product, and intermediate profiles which gradually transform this rough profile approximation into the final product profile by the time the extrudate exits the die.
While dies of the type mentioned immediately above do solve many of the problems associated with the extrusion of products formed of low melt strength thermoplastics having complex profiles, they typically cause difficulties during extruder start-up. That is, due to the increased axial length of these dies (necessitated by the gradual profile transformation of the extrudate within the die cavity), the operating pressure of the screw extruder may sometimes not be sufficient to force the extrudate through the die cavity. Obviously, manual assistance during start-up (as by physically pulling the extrudate through the die cavity from its outlet end) is not a solution since the terminal end of the extrudate is, during start-up, physically located within the die cavity and is therefore inaccessible.
What has been needed therefore, is a die assembly and/or method for extruding low melt strength thermoplastics to obtain products having relatively complex cross-sectional profiles. It is towards attaining solutions to the problems briefly mentioned above to which the present invention is directed.
According to the present invention, an assembly of elongate die spools is provided, each of which is formed of upper and lower die segments so as to be separable one from the other. Individual die spool cavities are therefore defined collectively by the upper and lower die segments of each die spool--i.e., one portion of the die cavity is defined by the lower die segment, with the other portion of the die cavity being defined by the upper die segment. A number of such individual die spools may be coupled together in an end-to-end fashion so that the individual die cavities of each communicate with one another and collectively establish an elongate die cavity for the die spool assembly along an extrusion axis. The die spool assembly (i.e., an assembly comprised of a number of individual die spools) may thus be mounted to a primary die associated with the screw extruder (or to the outlet of the screw extruder itself) so that the extrudate is transferred into and through the die cavity of the spool assembly along the extrusion axis.
The elongate die cavity of the die spool assembly may thus be configured so as to gradually shape the extrudate from a initial (rough approximation) cross-sectional profile to a final (or near final) cross-sectional profile for the product. That is, the upstream cavities of individual die spools in the assembly may provide an initial shaping of the extrudate, while the downstream cavities of individual die spools effect final (or near final) product shaping. Those cavities intermediate the upstream and downstream ones would therefore provide a gradual transition between the initial cross-sectional profile and the final (or near final) cross-sectional profile of the extrudate.
The "split body" die spools and their capability to be coupled to other similar die spools reduces (if not eliminates) many of the problems associated with extruder start-up. That is, during start-up, the lower die segment of an upstream-most one of the die spools may be coupled to a primary extrusion die associated with the screw extruder (or to the outlet of the screw extruder itself) so as to initially support the extrudate being discharged from the die (or extruder). When this length of extrudate approaches (or just begins to extend beyond) the downstream end of the lower die segment, a lower die segment of the next sequential die spool may then be mateably coupled to the downstream end of the lower die segment of the first die spool so as to support the continually increasing length of extrudate (i.e., due to its being continually advanced by means of the screw extruder and/or by means of manual pulling forces exerted upon the terminal end of the extrudate). Shortly thereafter, the upper die segment of the first die spool may be mated with its already positioned lower die segment (which is already supporting a length of the extrudate) so as to enclose the extrudate and thus force it through the die cavity collectively defined by the first die spool.
As will be appreciated, since the extrudate which extends beyond the downstream end of the first die spool is then being supported by the lower die segment of the next sequential die spool, its physical integrity is not compromised. Thus, the upper die segment of the next sequential die spool may then be mateably coupled to its lower die segment when the terminal end of the extrudate approaches (or extends just beyond) the downstream end of the lower die segment of the next sequential die spool. In such a manner, the next sequential die spool then defines another portion of the assembly's die cavity.
The technique referenced above may be repeated using additional die spools in sequence as may be necessary or desired for a given product. Hence, by employing the individual die spools in the manner briefly described, many of the problems associated with extruder start-up to produce products of complex cross-sectional profiles may be reduced (if not eliminated) since the extrudate may, at all times during start-up, be physically gripped and pulled along the extrusion axis to thereby assist in its conveyance along the extrusion axis.
The die spools of this invention are individually temperature-controlled, preferably by providing an electrical resistance type heating jacket (or any other equivalent temperature control means) around the die spool's body. Hence, as the extrudate is being advanced within the die spool cavities, the temperature of the extrudate at locations along the extrusion path (defined by the die spool cavities) may be controlled to facilitate start-up. And, during normal operating conditions, this temperature control ensures that the extrudate is shaped in the desired manner by the die spool cavities.
Further advantages and aspects of this invention will become more evident after considering the following detailed description of the preferred exemplary embodiments thereof.